Direct optical three-dimensional surveying of one or more teeth in the mouth of a patient can provide digital construction data for the computer-controlled production of dentures without using a dental impression. Such a three-dimensional representation, known as an “optical impression” in analogy to physical molding, can be made with one or more camera images without contacting the object. The camera can be freely guided for this operation like an angled handpiece or guided with one or both hands whilst supported on the teeth.
The method of measurement implements the principle of active triangulation, in which a single stripe of light or a striped pattern of parallel stripes of light is projected on to the object to be imaged by projecting means and the projected image is recorded by a two-dimensional camera at an angle of parallax.
By reason of the surface texture of the teeth the projected stripe is no longer a straight line but appears to be curved and displaced relatively to the straight path. The position of the lines of light can provide information on the surface texture of the object being imaged. Following the measuring procedure, the memory of the computer contains a digital three-dimensional data model of the imaged object, which can be displayed, for example, on a monitor as a still video picture or can serve as the basis for the computer-controlled production of a denture.
In order to increase accuracy, use may be made of the phase-shifting triangulation method. This method involves the successive production of a number of basic images at various positions of the phase relationship of the grid, and from these images a phase related image is computed. From the phase related image it is then possible, with the aid of calibration data, to compute a relief image of the object. Various algorithms for the computation of the phase related image from the individual basic images are known. For example, three, four, or five basic images showing a relative phase shift of 120° or 90° can be recorded. It is also possible to compute a contrast image from the basic images. For example, a contrast image can be computed from four basic images having intensities I1 . . . I4 according to the formula:K=√((I1−13)2+(I2−I4)2).
The phase-shifting method involves, on account of tolerances or systematic sources of error, typical periodic disturbances or noise. These periodic disturbances occur in the phase related image and also in the contrast image or relief image at a multiple of the grid frequency.
For example, any deviation from the linear motion of the optical grid leads to noise at 2-fold grid frequency, any change in the intensity of the grid illumination to noise at 1-fold grid frequency, and deviations from the linearity between the light signal and electrical signal leads to noise at 4-fold grid frequency.
In general, the computing algorithms become less prone to noise generation as the number of recorded basic images increases. Nevertheless, the described periodic disturbances are expressed to various degrees despite the distinctly more elaborate treatment.
The quality of the phase related image and contrast image may be increased, for example, by effecting filtration. This must mean a loss of resolution however. Improvement in quality caused by averaging a plurality of, phase related images or contrast images suffers on the other hand from the drawback of a distinctly prolonged recording time.
It is further possible to effect control over the recording of the basic images in a very precise manner such that the periodic disturbances occur to a lesser extent. However, such precise control involves elaborate means. In addition, external conditions, such as temperature, moisture, or the force of gravity, and also long-term effects have an influence on the imaging process and the image quality obtained.
These problems are solved by the present invention. It is an object of the invention, as characterized in the claims, to provide a method of the above type for creating an image of a an object, which method affords much greater measuring accuracy than known techniques whilst involving comparatively little elaboration.